1. Field of the Invention
The present invention relates to radar apparatuses and, more particularly, to a radar apparatus for use in automobiles.
Under recent circumstances, in which automobile accidents are increasing yearly with the increase in use of automobiles, it has become necessary for an automobile to be equipped with an apparatus supporting safety of operation. Such an apparatus should be capable of warning a driver of a possible crash, measuring a ground speed accurately and controlling the engine properly while the automobile is driven on a wet road.
2. Description of the Related Art
Conventional radars for measuring a relative speed and distance of an automobile with respect to a target object include a frequency modulated continuous wave (FMCW) radar and a pulse Doppler radar. In an FMCW radar, as shown in FIG. 1, a triangular baseband signal is supplied to a transmission voltage controlled oscillator (VCO) 10 for frequency modulation. A resultant high-frequency signal obtained from the voltage controlled oscillator 10 is transmitted via a transmission antenna 11. A portion of the high-frequency signal is branched off to a reception mixer (MIX) 12 supplied with a received signal received via a reception antenna 13. According to this arrangement, a beat signal commensurate with a distance from a target object and a relative speed with respect to the target object is obtained. Since such a construction enables a significantly simple signal processing unit to provide a relative speed signal and a distance signal, it is extensively being applied to radar apparatuses such as an automobile radar apparatus which are expected to be small and inexpensive.
In one specific implementation of a radar apparatus illustrated in FIG. 2, a switch (SW) 16 is provided between the reception antenna 13 and the reception mixer 12 so as to prevent a signal-to-noise ratio from being degraded due to a leak of the triangular modulating signal to the reception side and to attenuate 1/f noise provided by the reception mixer (MIX) 12. The switch 16 may be operated according to a drive signal (LO) from an oscillation source 18 so as to subject the received signal to frequency conversion. The received signal output from the reception mixer (MIX) 12 is supplied to a second mixer 22 via a band-pass filter (BPF) 20. In the second mixer 22, the received signal is subject to frequency conversion again, using the drive signal (LO), so that a beat signal is obtained.
For example, a frequency f.sub.0 of a transmitted signal output by the voltage controlled oscillator 10 is on the order of 10-100 GHz, a frequency f.sub.SW of the switch drive signal (LO) is on the order of 10-100 MHz and a beat frequency f.delta. produced by the transmitted signal and the received signal is below the order of 10-100 KHz. The frequency of the triangular wave is on the order of 100-1000 Hz. The amplitude of the transmitted signal supplied to the reception mixer 12 varies due to the frequency response of the VCO 10 and the line. The frequency of the amplitude variation is 100-1000 Hz. Since the reception mixer 12 has an AM demodulating function as well as the frequency conversion function, the output signal of the reception signal 12 inevitably includes noise in the form of the amplitude variation.
The frequency of the received signal received via the reception antenna 13 is given by f.sub.0 +f.delta.. As a result of the frequency conversion using the switch 16, a received signal having frequencies between f.sub.0 +f.delta.-f.sub.SW and f.sub.0 +f.delta.+f.sub.SW is produced. Accordingly, the reception mixer 12 produces an IF signal having frequencies between f.sub.SW -f.delta. and f.sub.SW +f.delta.. The IF signal contains a noise in the form of amplitude variation having a frequency on the order of 100-1000 Hz. Since the IF signal has a frequency on the order of 10-100 MHz, the noise in the form of the amplitude variation is removed by the band-pass filter 20. The mixer 22 subjects the IF signal having the noise removed to frequency conversion so as to obtain the beat signal having a frequency f.delta..
The antennas 11 and 13 must have a size commensurate with a beamwidth capable of covering a single traffic lane so as not to detect a reflected wave from a non-target object outside a detection range of the radar. For example, given that the detection range is 100 m, and the width of a lane is 3.6 m, the beamwidth required to keep out a reflected wave from a vehicle driven in another lane 100 m ahead would be 2.1 degrees. As is well known, the relationship between the beamwidth B and the diameter D of the reflector antenna is given by EQU D.apprxeq.70.times..lambda./B
Assuming that the beamwidth is 2.1 degrees and the frequency is 60 GHz (the wavelength=5 mm), the diameter D of the antenna is 167 mm, according to the above equation.
In the 100 m distance, an area covered by the radar is determined by a product of gain drops of the transmission and the reception antenna at a given angular separation. Thus, the diameter may actually be smaller than the result obtained according to the above equation. Antennas having a diameter of about 110 mm are usually used.
Desirably, an automobile radar apparatus is designed so as not to detract from the aesthetic design of the automobile body. For optimum matching with the design, the smaller the radar apparatus, the better. Also, an automobile radar apparatus is ideally inexpensive in order to be available to the general public. However, requirement for concurrent transmission and reception imposes the use of both the transmission antenna and the reception antenna, so that it is difficult to achieve reduction of the size and price of an automobile radar apparatus according to the conventional technology.
As shown in FIG. 3, one approach to reduce the size of the radar apparatus is to use a directional coupler 26 so that a transmission and reception antenna 24 can be shared. In the circuit of FIG. 3, a portion of the transmission power is supplied to the transmission and reception antenna 24 via the directional coupler 26. An isolation capability of the directional coupler 26 is utilized to supply a local power to the reception mixer 12. The degree of coupling of the directional coupler 26 is set below 10 dB so as not to draw an excessively large power to the reception mixer 12 in consideration of the withstand capability of the reception mixer 12. Accordingly, a problem with this approach is that the voltage controlled oscillator 10 is required to output a significantly larger power than the actual output power needed to attain a satisfactorily long detection range. For example, assuming that the power actually radiated by the antenna is 10 mW, a power exceeding 100 mW is needed. Although an output on the order of 10-100 mW can be easily obtained by high-output devices such as an IMPATT diode and a Gunn diode in the milliwave band, an output exceeding 100 mW cannot be obtained easily by the present device technology. It is also to be noted that devices such as the IMPATT diode and the Gunn diode are quite expensive.
Since the isolation capability of the directional coupler 26 is used in the approach of FIG. 3, the local input power varies due to the antenna impedance. For this reason, it is difficult for the reception mixer 12 to perform stable frequency conversion.
One problem that arises as the number of vehicles equipped with an automobile radar apparatus increases is radio interference from other vehicles. Since automobile radars share generally the same frequency band, radars carried on automobiles driven in the opposite lane may cause radio interference so that the target object ahead of the user driving in the same lane, becomes lost on the radar.
Conventional approaches to prevent radio interference include the use of a 45-degree linearly polarized wave antenna or a circularly polarized wave antenna. Polarization discrimination provided by such types of antennas is used to attenuate radio interference from other vehicles. However, polarization discrimination provided by such types of antennas is 20-25 dB at best. As the number of vehicles equipped with radar increases, the interfering signal level increases so that the conventional technology cannot provide satisfactory performance from the radar.